In the manufacture, for example, of urethane foams the foaming and polymer-forming reactions take place simultaneously, when excess isocyanate is mixed with a polyol. Similarly in the manufacture of epoxy the pot time, that is the amount of time between when the components are mixed and when they set, is very short. Thus it is necessary to mix these reactant components at the last possible moment before injecting the mixture into the mold cavity. Hence they are normally mixed right in a mixing chamber in the nozzle that itself is directly and closely connected to the mold.
Such a nozzle, as described in German patent document No. 2,007,935 of R. Keuerleber and F. Pahl is of three-part structure. Two generally identical outer parts or housing halves are connected to respective supplies of the components and have feed passages opening at confronting housing faces between which the third part of the housing can slide. This central third part is formed internally with the mixing chamber and with two injection passages or ports that open inward into the mixing chamber and outward on the third part at faces thereof that lie flatly against the respective faces of the housing halves. In a feed position of this slider each feed passage is aligned with the respective injection passage and opens therethrough into the mixing chamber. In a return position the slider blocks the outer ends of the injection passages and aligns a return passage connected back to the respective supply with each feed passage, so that there can be flow through the nozzle even as the batch is being extruded into the mold and there is no flow into the mixing chamber.
Similarly, in German patent No. 2,515,579 of P. Hartwig the slider is provided on its faces with a groove that forms a passage between the feed and return passages in the return position.
Such arrangements are fairly difficult to seal. The sliding joint must be perfectly machined and even so is very difficult to seal properly. In addition these devices often take up quite some room, making it impossible to fit them in cramped quarters, which are often all that is available in a molding operation. Furthermore these devices are relatively complex and therefore quite expensive to build and maintain.